This invention relates to an electrosurgical instrument and an electrosurgery system including the instrument and a radio frequency generator system for treatment of tissue, wherein the operative site and distal portion of such an instrument are immersed in an aqueous solution during use to vaporise, coagulate, desiccate or otherwise thermally modify such tissues.
Electrosurgery has been used in surgical practice for over fifty years and during that time has seen a number of improvements to enhance safety and performance in more challenging surgical environments. One such challenge has been the emerging use of minimal access or endoscopic techniques to perform an ever increasing range of surgical procedures.
Traditional monopolar and bipolar electrosurgical devices have been widely used in endoscopic surgery but have suffered from a number of technical disadvantages, particularly when the operative site is distended or irrigated with aqueous solutions. Such solutions are commonly employed to improve endoscopic visualisation and are usually introduced to the body cavity through a specific channel provided in the endoscope itself. The immersion of electrosurgical instruments in aqueous solutions introduces a number of technical hurdles in terms of insulation and preventing power dissipation to the solution rather than the target site.
Monopolar electrosurgery requires the aqueous solution to be non-conductive in order to operate efficiently. Despite use of solutions such as Dextrose, Sorbitol and Glycine which have very low values of electrical conductance, tissue and bodily fluids released during the operation can significantly increase the conductance of fluids bathing the target site. This usually necessitates increasing power output to overcome losses to the solution in order to maintain performance. Increasing power output increases the recognised risk of inadvertent burns to the patient or operator when using monopolar arrangements.
Burns associated with monopolar arrangements can be avoided, albeit in an instrument with limited power, by use of capacitive coupling in a return path between tissue adjacent the operative site and the conductive casing of a self-contained battery-powered instrument such as that disclosed in the applicant""s International Application No. WO97/15237. This device has a single exposed electrode and with an integral generator inside a metallic handheld casing and is intended for dry field use at high frequencies. The casing, acts as a conductor capacitively coupled to its surroundings to provide a capacitive return path from tissue being treated to the generator within the casing. The impedance of the return path is widely variable due to the varying degree to which the instrument is capacitively coupled to the surgeon""s hand and the variable juxtaposition of surrounding conductive masses. A thin plastics coating is provided over the casing to prevent direct electrical contact with conductive masses.
While a conventional bipolar electrosurgical instrument, in which the two poles of the electrosurgical output are mounted as neighbouring electrodes on the tip of the instrument and both are required to contact tissue in order to produce an effect, to a large extent eliminate the risks of burns, the output power will still preferentially pass through conductive fluid present at the application site rather than the tissue itself. Nonetheless, the heating of such fluid can produce secondary heating of the tissue and thereby produce coagulation or desiccation of the tissue. The problem is much more evident when trying to use a bipolar arrangement to cut or ablate tissue whereby the solution lowers the impedance of the output and prevents the output voltage reaching the levels necessary to induce vaporisation of the tissue, typically requiring a peak-to-peak voltage in excess of 500V.
U.S. Pat. No. 5,009,656, Reimels, describes a method of overcoming this problem in which the two electrodes of a bipolar pair are brought sufficiently close, such that the gap between them supports the production of water vapour and direct arcing between the two electrodes. Although the technique provides for a method of arcing during immersion in conductive fluid, the gap is so small that the tissue effect is very limited.
U.S. Pat. No. 4,116,198, Roos, describes a further technique of overcoming some of the problems associated with use of bipolar arrangements when immersed in electrically conductive solutions. This technique overcomes the limitations of Reimels by use of a return electrode which is set back from the active electrode such that a more protracted electrical circuit is completed by the solution and direct arcing between the two is avoided. By this means only one electrode of the bipolar pair is required to contact tissue and the electric field is sufficient to include the tissue at the application site. This technique was successfully employed in endoscopic surgical procedures involving transuretheral resection of the prostate gland, as described by Elsasser and Roos in an article entitled xe2x80x98Concerning an Instrument for Transuretheral Resection without Leakage of a Currentxe2x80x99, published in the German magazine xe2x80x98Acta Medico Technicaxe2x80x99 1976 vol. 24 No. 4 pages 129-134.
The present applicants"" co-pending patent applications relating to electrosurgery describe further improvements over the Roos prior art to control the dimensions of the active electrode relative to a complex interaction between the configuration of the active electrode, the power threshold to establish arcing at the active electrode and control features in the electrosurgical generator component of the system. The commercial embodiments of these inventions are now widely used in the fields of arthroscopic, hysteroscopic and urological surgery.
U.S. Pat. Nos. 5,366,443 and 5,697,909 in the names of Eggers and Thapliyal describe an alternative approach using an array of active electrodes which can be selectively activated, or current limited, in order to reduce power dissipation into the electrically conductive fluid.
The above arrangements tend to suffer from a number of problems and technical limitations during use. Carbon tracking is a problem wherein the carbon residue derived from tissue vaporisation forms a conductive track between active and return electrodes. Once established, the track has a negative temperature co-efficient of resistance so that the hotter the carbon becomes, the more conductive it becomes and the more current flows along the track between the electrodes. The temperature developed in the track places huge thermal stresses on the insulator separating the return and active electrodes, which may result in catastrophic failure. Similar failures can occur due to overheating of the electrode assembly without carbon track formation when only portions of the distal tip assembly are in contact with fluid such that current can still flow in the absence of sufficient fluid cooling of the assembly.
In these prior devices, the electric field becomes concentrated in the region representing the shortest conduction path through the fluid medium. The shortest paths occur between the most distal portion of the return electrode and the most proximal region of the active electrode exposed to the fluid. The effects of this are two-fold: firstly, the high current density may cause the return electrode to become xe2x80x9cactivexe2x80x9d, particularly when the distal portion of the electrode is only partially immersed in fluid due to the accumulation of gaseous by-products produced during vaporisation; and, secondly, the depth of tissue effect is limited by the concentration of the electric field, particularly when a deeper coagulative effect is desirable.
When the active electrode is partially wetted and partially enveloped in vapour, very high powers are required to sustain what is, in effect, an unstable condition which usually results in intermittent collapse of the vapour pocket and variable surgical performance. The prior art describes certain ratios of the shortest and longest conduction path lengths between the active and return electrodes to reduce this effect. The effect is exacerbated by high fluid flows when the vapour pocket can become quenched, and, once again, high powers are required to overcome this quenching effect. Nevertheless, the surgical performance is variable. The prior art includes techniques to reduce this power threshold of vaporisation in the presence of a fluid flow, but the techniques involved impose some geometric limitations on the electrode assembly which can be difficult to implement in certain fields of application, particularly when active suction or fluid delivery occurs adjacent to, or through, the electrode itself.
It is an aim of this invention to provide an electrosurgical instrument for treating tissue immersed in a conductive fluid, which overcomes at least some of the disadvantages set out above.
According to a first aspect of this invention, there is provided an electrosurgical instrument for use at a frequency or frequencies in the range of from 100 kHz to 50 MHz, the instrument having a bipolar electrode assembly for tissue treatment when immersed in a conductive fluid, wherein the instrument comprises an elongate shaft mounted at one end to a handpiece, and carrying the electrode assembly at its other end, and wherein the electrode assembly comprises at least one distal active electrode, and an adjacent return electrode set back proximally of the active electrode, the return electrode being encased in an insulative dielectric layer. When the electrode assembly is immersed in a conductive fluid, the return electrode, due to having no exposed portion immersed in fluid, is capacitvely coupled to the fluid, the capacitance of the coupling depending on, inter alia, the thickness of the dielectric covering, its relative dielectric constant (xcex5r), and the area of the return electrode.
According to another aspect of the present invention, there is provided an electrosurgical system including a generator delivering a radio frequency output for the treatment of tissue structures in the high to very high frequency range. The generator is connected to an electrode assembly immersed in an electrically conductive fluid, the electrode assembly including an active zone and a return zone at the distal end, wherein one of either or both zones are insulated from direct electrical contact with either the tissue or the electrically conductive fluid such that, in use, the output circuit is completed by dielectric coupling through the insulation.
The invention also includes an electrosurgical instrument for wet field electrosurgery, wherein the instrument comprises a tubular elongate shaft for insertion into a body cavity and, at the end of the shaft for insertion into the body cavity, an electrode assembly comprising a first conductor at an extreme distal end of the instrument and a second conductor insulated from the first conductor and set back from the distal end, wherein at least the second conductor is encased in an insulative outer dielectric layer. An electrosurgery system including such an instrument may include a generator having a pair of output terminals coupled respectively to the first and second conductors of the electrode assembly, wherein the frequency of operation of the generator and the construction and materials of the second conductor and the encasing insulative layer are such that when a radio frequency current of 2 amps is delivered to the electrode assembly when immersed in normal saline, the current density at the outer surface if the insulative layer does not exceed 50 mA/mm2.
According to another aspect of the invention, there is provided an electrosurgery system for treating tissue structure contained within a body cavity and immersed in an electrically conductive fluid, the system comprising: a generator for delivering a radio frequency output for the treatment of tissue structures in the high to very high frequency range; and an electrode assembly connected to the generator, the electrode assembly comprising: an active electrode including an active zone at a distal end of the active electrode; a return electrode including a return zone defined by an electric field developed between the active electrode and the return electrode when the electrode assembly is connected to the radio frequency output of the generator; and an insulator separating the return electrode from the active electrode; the return zone being coated with a layer of dielectric insulating material wherein, in use, the return zone is insulated from direct electrical contact with the tissue and the electrically conductive fluid so that an electrical circuit is completed between the active and return zones by dielectric coupling through the dielectric insulating material.
According to yet another aspect of the invention, there is provided an electrode assembly for treating a tissue structure contained within a body cavity and immersed in an electrically conductive fluid, the electrode assembly being connected to a generator for delivering a radio frequency output for the treatment of tissue structures in the high to very high frequency range, wherein the assembly comprises: an active electrode including an active zone at a distal end of the active electrode; a return electrode including a return zone defined by an electric field developed between the active electrode and the return electrode when electrode assembly is connected to the radio frequency output of the generator; and an insulator separating the return electrode from the active electrode; the return zone being coated with a dielectric insulating material layer wherein, in use, the return zone is insulated from direct electrical contact with the tissue and said electrically conductive fluid so that an electrical circuit is completed between the active and return zones by dielectric coupling through the dielectric insulating material. The active zone may be coated with a second dielectric insulating material layer so that the active zone is insulated from direct electrical contact with the tissue and the electrically conductive fluid and, in use, a circuit is completed between the active and return electrodes by dielectric coupling also through the second dielectric insulating material.
According to a further aspect, the invention also includes an electrode assembly for treating tissue structure contained within a body cavity and immersed in an electrically conductive fluid, the assembly being connected to a generator for delivering a radio frequency output to said electrode assembly, wherein the assembly comprises: an active electrode in the form of a ceramic body having an internal cavity lined with metal, the active electrode including an active zone at a distal end of the active electrode; and a return electrode including a return zone defined by an electric field developed between the active electrode and the return electrode when the electrode is connected to the radio frequency output of said generator; the ceramic body being made of a dielectric insulating material, wherein, in use, an electrical circuit is completed between the active and return zones by dielectric coupling. The radio frequency output advantageously interacts with the dielectric insulating material to result in a substantially uniformly distribution of the electric field over the return zone and a corresponding substantially uniform current density over the return zone. Preferably, the return zone is coated with a second dielectric insulating material, and the radio frequency output interacts with the dielectric insulating material and the second dielectric insulating material to result in a substantially uniformly distribution of the electric field over the active and return zones and a corresponding substantially uniform current density over those zones.
Features of other aspects of the invention and its preferred embodiments are set out in the claims.
The properties of the dielectric insulating material, coupled with variations in the electrosurgical output frequency and the combination of one or either or both of the active and return zones being coated in the dielectric material, enable manipulation of the electric field and current density created during use in order to confer significant performance advantages over prior art devices. These advantages can be realised in the electrodes geometrically configured to perform specific procedures on different tissue structures such as structures contained within cavities, lumens, ventricles or other natural body cavities containing an electrically conductive fluid, whether or not such fluid is naturally present or introduced as part of the surgical procedure. Such cavities may also be provided by artificial fluid enclosures or by surgical creation of the cavity. Of specific advantage, the performance of instruments in accordance with the invention can be made less susceptible to overheating and the effects of fluid flow on during use.
The invention will be described below in more detail by way of example.